The invention relates to a coaxial cavity resonator, the resonance frequency of which can be adjusted.
Cavity resonators are generally used in communications networks for making filters, especially when the power of the signal to be transferred is relatively large. This is due to the fact that losses caused by such resonator filters are small, which means only a slight attenuation of the effective signal. Additionally their response characteristics are easy to control and set even according to strict specifications.
In most filters, both the location and width of the pass band of the filter is meant to be fixed. In some filters the width of the pass band of the filter is meant to be constant, but the location of the pass band must be possible to select from within a certain total area. Thus an adjustment possibility for moving the pass band is needed in the filter in addition to the basic tuning.
The structure of a coaxial cavity resonator, or shorter a coaxial resonator, comprises an internal conductor, an external conductor consisting of the side walls, a bottom and a lid. The bottom and the lid are in galvanic contact with the external conductor, and all three together form a closed resonator casing. Usually the lower end of the internal conductor is galvanically connected to the bottom and the upper end is in the air, whereby the transmission wire formed by the resonator is short-circuited in its lower end and open in its upper end.
The adjustment of the resonance frequency of the resonator is usually based on the fact that the capacitance between the internal conductor and the lid is changed, whereby also the electric length and resonance frequency of the resonator change. Metallic tunings screws placed in the lid of the resonator are generally used as adjustment parts. When turning the screw its distance from the internal conductor of the resonator changes, as a result of which the capacitance between the internal conductor and the lid changes. A disadvantage with using tuning screws is that the screw accessories increase the number of parts of the filter and the threaded screw holes mean an increase in work stages and thus an increase in manufacturing costs. Additionally the electric contact in the threads can deteriorate over time, which causes changes in the tuning and an increase in losses in the resonator. In high capacity filters there is also a danger of breakthrough, if the tip of the screw is close to the end of the internal conductor. The tuning thus takes time and is relatively expensive.
U.S. Pat. No. 6,255,922 presents a resonator, which has a moveable piece for adjusting the resonance frequency, which piece is made from a non-conductive material, which piece is moved inside a resonator piece made from a ceramic material. The structure is quite complicated, and it is not able to provide very large adjustments in the resonance frequency.
U.S. Pat. No. 7,474,176 introduces a cavity resonator, where the size of the internal conductor of the resonator is changed by moving an adjustment piece inside the internal conductor with a screw arrangement penetrating the resonator. This structure is also complicated and only able to make fine adjustments in the resonance frequency.